1. Field of the Invention
The present invention relates to a wire dot type printer having dot pins of a print head being different in gaps thereof with respect to a platen from each other, and more particularly to a dot drive circuit for controlling the drive time interval of the dot pin.
2. Description of the prior Art
FIG. 2 is an illustration which shows a distance relation between a printing paper and a dot pin. As shown in the figure, a wire dot type print head 1 (hereinafter simply referred to as a print head) is diposed in a confronting relation with a platen 2. Dot pins 3 of the print head 1 respectively have a different gap to the platen 2, and they provide a maximum gap L between the tip ends of the uppermost end and the lowermost end of the dot pins 3 and a printing paper 4 placed on the platen 2. The print head 1 employed here is of a 9-pin head.
Conventionally, the dot pin 3 of the print head 1 is driven by a drive circuit shown in FIG. 5. The drive circuit of FIG. 5 is a prior example. As shown in the figure, designated at 5 is a microprocessor, etc., which issues a timing signal A for driving the dot pin and selection signals B.sub.1, through B.sub.9 each for selecting the dot pin in concern driven in certain drive timing. Designated at 6 is a monostable multivibrator (hereinafter referred to as monostable), which receives the drive timing signal A and thereby generates a drive pulse C. Designated at 7.sub.a through 7.sub.i are respectively gate circuits, each of which switches on transistors 8.sub.a through 8.sub.i respectively corresponding to the selection signals B.sub.1 through B.sub.9 when the monostable 6 issues the drive pulse C, for conducting a current through magnets 9.sub.a through 9.sub.i for printing.
FIG. 6 is a timing chart illustrating the dot drive circuit of FIG. 5. Here, the timing chart is for a signal corresponding to an odd-numbered dot pin 3 for brevity of the description. As shown in FIG. 6, when the microprocessor 5 shown in FIG. 3 issues selection signals B.sub.1, B.sub.3, B.sub.5, B.sub.7, and B.sub.9 for example and furthermore issues the drive timing signal A, the monostable multivibrator 6 issues the drive pulse C during a preset dot drive time interval T. With this output signal, the transistors 8.sub.a, 8.sub.c, 8.sub.e, 8.sub.g, 8.sub.i gets an on-state during the time interval T to operate the magnets 9.sub.a, 9.sub.c, 9.sub.e, 9.sub.g, and 9.sub.i for printing.
Thereupon, the dot drive time interval T is set to apply sufficient impact force to the dot pin 3 even if the moving distance of the dot pin 3 is the maximum gap L of FIG. 2.
However, such a prior technique has a gap between a central dot pin and the platen made narrower as compared with the maximum gap L, so that the central impact force becomes excessive and hence results in uneven printing density as well as stains due to the dot pin being rebounded.